The standard arrangement for implementing a surface-based multi-electrode resistivity (MER) array involves the insertion of metal probes a few inches into the top surface of the ground. The metal probes are devices such as electrodes or conductors. The information obtained from these arrays functions as a resistivity sounding of the local underground electrical resistance. The resistivity soundings are represented as resistivity data points, which provide a model for the bulk measurement of the soils and rocks in the subsurface. Any variety of subsurface geologic features can be detected or identified on the basis of an analysis of the resistivity model, although limited by the resolution available with the data.
The electrodes are typically connected to an insulated, multi-core cable, which is connected to a switch box for regulating the flow of electricity. A computerized resistivity instrument records and displays the resistivity soundings, typically in the form of resistivity data points. The resistivity data points are then combined to create a model of the subsurface. A laptop is a convenient appliance for implementing the computerized resistivity instrument.
A variety of electrode configurations are used to construct the individual arrays, such as a four electrode combination. In this array arrangement, one pair of electrodes functions as current electrodes since they are used to introduce a direct current into the earth. The other pair of electrodes functions as voltage or potential electrodes since they are used to measure the voltage. This voltage measurement is indicative of the local subsurface resistivity. Surface multi-electrode resistivity arrays employ principles similar to standard Ohmmeters used to test electrical circuits.
One performance metric of the resistivity imaging technique is the depth to which the resistivity sounding activity can reach. A greater depth of investigation into the subsurface is possible by increasing the distances between the electrodes. A protocol generally observed in electrical resistivity procedures is that the depth of investigation is approximately one-fifth (⅕) of the total length of the entire electrode array system positioned on the surface. The electrode arrays are typically organized along a survey line, making it easy to calculate the array length. For example, a one-hundred (100) foot array equals approximately a twenty (20) foot depth of investigation. However, while greater electrode spacing does yield a greater depth of investigation, this benefit comes at the cost of sensitivity and spatial resolution. As the array length increases, the resolution decreases, especially at the lower subsurface levels, because electrical current dissipates more widely as it penetrates deeper into the subsurface.
A typical model for representing the resistivity profile includes an inverted triangle having a wide, uppermost side lying directly at the surface, with the other sides tapering inwards and downwards to a terminal vertex. The resistivity data points are distributed throughout the interior of the triangle and represent the location-specific spatial distribution of the resistivity values in the subsurface region. The upper part of the inverted triangle proximate to the surface includes a greater amount of resistivity data points relative to the lower part of the triangle, due to the depth dependent changes in current dissipation. The resolution of any particular region within the inverted triangle model is a function of the number of resistivity data points contained within the area of interest. Attempts to increase the depth of penetration of the resistivity sounding measurements by increasing the array length have the adverse effect of reducing the overall resolution, since current dissipation is greater. However, opposite attempts to enhance resolution by increasing the density of the resistivity data points require a shortening of the array length, which reduces the depth of penetration and narrows the inverted triangle, resulting in a reduced spread of resistivity data points that compromises the ability of the imaging technique to acquire and detect targets. Accordingly, there is a tradeoff between obtaining the deepest possible resistivity data, while simultaneously trying to yield maximum resistivity data points for optimum resolution.
Based on the collection of resistivity data points represented in the inverted triangle model, an analysis of this profiling model enables the mapping and identification of geologic features, depending on whether the features exhibit higher or lower resistivity values compared to their surroundings. In this way, the presence and form of geologic features can be inferred from the resistivity profile model. For example, a stone foundation might impede the flow of electricity, while the nearby organic deposits might conduct electricity more easily than surrounding soils. Although used in archaeology for plan view mapping, resistance methods nevertheless have a limited ability to discriminate depth and create vertical profiles. Other applications of surface multi-electrode resistivity arrays include the measurement of the electrical resistivity of concrete to determine the corrosion potential in concrete structures.
Surveys to perform resistivity imaging can use any type of electrode array. One common example is the Wenner probe array, which is a linear array of four probes. The Wenner array is arranged current-voltage-voltage-current, at equal distances across the array. Electrodes are mounted on a rigid frame or placed individually. While quite sensitive, this array has a very wide span for its depth of investigation, leading to problems with horizontal resolution. A number of other array configurations attempted to overcome the shortcomings of the Wenner array. One of the most successful of these is the twin-probe array, which has become the standard for archaeological use. The twin-probe array has four probes: one current and one voltage probe mounted on a mobile frame to collect survey readings, and the other current probe placed remotely along with a voltage reference probe. These fixed remote probes are connected to the mobile survey probes by a trailing cable. This configuration is very compact for its depth of investigation, resulting in improved horizontal resolution. The logistical advantage of the more compact array is somewhat offset by the trailing cable. Additionally, a disadvantage of the twin-probe array is a relatively slow rate of survey, since the acquisition of a series of readings at different surface locations requires the mobile frame that houses the array to be moved to each successive location before a new reading can be taken.
An improvement on the slow rate of survey from the twin-probe array has come from the utilization of the wheeled array. The wheeled array uses spiked wheels or metal disks as electrodes, and may use a square array, which is a variation of the Wenner array, to avoid the encumbrance of a trailing cable. Wheeled arrays may be towed by vehicles or driven by human power. Although wheeled arrays do speed up the rate of survey, they cannot always be deployed in crowded urban areas since they require a wide range of mobility.
There are other versions of the standard linear electrode array besides the Wenner array. Systems that have long linear arrays of multiple electrodes are often used in geological applications, and less commonly in archaeology. These longer linear arrays take repeated measurements, often computer controlled, using different electrode spacings at multiple points along the extended line of probes. Data collected in this way may be used for tomography or for generating vertical profiles. Another type of array, different than a resistance based system, can utilize a capacitively coupled interface that does not require direct physical contact with the soil. These systems are capable of tomographic studies as well as mapping horizontal patterning. These capacitive coupling systems may also be used on hard or very dry surfaces that otherwise preclude the type of electrical contact that is necessary for probe resistance systems. While these systems show promise for archaeological applications, currently available systems operating on this capacitive coupling principle lack sufficient spatial resolution and sensitivity.
Accordingly, there remains a need in the art for a method of investigating subsurface geologic targets that employs a multi-electrode resistivity array and that overcomes the drawbacks and limitations of conventional surface-based array arrangements, with the additional objective that the method facilitates the collection of enhanced electrical resistivity data to improve the acquisition, detection and identification of the subsurface targets.